The present invention relates generally to a sensor arrangement which is based on the use of a FET (Field Effect Transistor) which responds to changes in distance from a substrate that emits an electric field and which, due to its rapid response characteristics, finds application in at least Contact Probe Storage (CPS) arrangements.
It has been hitherto been proposed to sense data which is written onto a movable medium using a probe that is supported on a cantilever and used to contact the medium. By using heat transfer characteristics between the movable medium and the probe (or a portion of the cantilever), it is possible to determine minute changes in distance between the movable medium and the cantilever on which the probe is carried, and use this as a means for reading out the data stored on the movable medium.
In the above type of arrangement, both imaging and reading are carried out using a thermomechanical sensing concept. A heater in the cantilever that is used for writing is also used as a thermal readback sensor by exploiting a temperature-dependent resistance function. That is to say, in this type of arrangement, the resistance (R) increases nonlinearly with heating power/temperature from room temperature to a peak value of 500-700° C. The peak temperature is determined by the doping concentration in the heater platform, which ranges from 1×1017 to 2×1018. Above the peak temperature, the resistance drops as the number of intrinsic carriers increases because of thermal excitation.
For sensing, the resistor is operated at about 200° C. This temperature is not high enough to soften the polymer medium, as is necessary for writing, but allows the molecular energy transfer between the cantilever on which the probe is carried, and the moving medium, to remove heat and thus provide a parameter which allows the distance between the cantilever on which the probe is carried and the medium on which the probe is running to be measured.
That is to say, this thermal sensing is based on the fact that the thermal conductance between the heater platform and the storage substrate changes according to the distance between them. The medium between a cantilever and the storage substrate, in this case air, transports heat from the heater/cantilever to the storage media/substrate. When the distance between heater and sample is reduced as the probe moves into a bit indentation, heat is more efficiently transported through the air and the heater's temperature and hence its resistance decreases. Thus, changes in temperature of the continuously heated resistor are monitored while the cantilever is scanned over data bits, providing a means of detecting the bits.
Under typical operating conditions, the sensitivity of the thermomechanical sensing is even better than that of piezoresistive-strain sensing inasmuch as thermal effects in semiconductors are stronger than strain effects. A ΔR/R sensitivity of about 10−4/nm is demonstrated by the images of the 40-nm-size bit indentations. This is better than the results are obtained using the piezoresistive-strain technique.
Nevertheless, the thermal response has been found to be slower than desired and is significantly slower than the cantilever's ability to mechanically follow the data pattern written in the medium. This leads to the system's read performance being slower than it would be if it were not limited to the thermal response of the sensing system.